Transcript Ecology and Disturbance

Why does succession take so
long?
• Different plant species have different
ecological requirements. A beech or liveoak needs shade as a seedling.
• They also need soil moisture which means
the soil must have a high organic content.
• So succession is also the development of
soil and colonization by soil organisms.
3 models of succession
Connell and Slatyer (1979)
Facilitation
• Initially thought that all succession was due to facilitation.
Example of facilitation: shade provided by pines allows
seedlings of broad-leaved trees to survive.
Or
Growth of a nitrogen-fixing plant on sandy (nutrient poor)
soils such as alder enriches the soil sufficiently for other
species to colonize.
Tolerance
• All species could live at all stages of the succession, but
differing dispersal abilities/adaptations ensures earliest
stages occupied by pioneer-type species.
• As succession proceeds fewer and fewer of the early
successional species can tolerate the new conditions and so
the system matures toward D.
• Dispersal distance is therefore also a big factor to be
considered.
Inhibition
• Species mutually inhibit one another
through competition. System can only
change when an individual dies an is
replaced.
• What will influence that replacement?
Soil maturity and succession
• Soil accumulates
organic matter as
succession
proceeds.
• Increased ability
to hold moisture.
• Pioneer species
are shaded out.
The balance
• Now understood that facilitation, tolerance,
and inhibition all combine to produce
succession.
Clements vs Gleason
• We now know that species respond
individualistically to change.
• Communities are not superorganisms and
will not always return to a predictable
equilibrium.
Primary Succession:
Colonization
of new
Colonization
of areas
new areas
Colonization of a new area
• Follows succession from pioneers to
competitors…..but all have to disperse
there.
• Distance from source is important…can
larvae survive long enough to be
transported there? Can seeds be blown
there? Can mammals swim there? Can birds
fly there?
Two ways to study succession
• Follow one location from disturbance to
maturity, ex. Krakatau, Mt St Helens.
• Select similar habitats at different times
since similar disturbance, ex. Glacier Bay,
building riverbank
Intertidal
succession
Ulva
Sea lettuce
• Macroalgal succession
over 30 months on
experimental concrete
blocks.
Primary succession in Glacier
Bay, Alaska
• Steadily retreating
glacier since 1850s.
• New land surface
revealed…primary
succession.
• Oldest succession
where ice first
retreated.
1912
1850
Glacier Bay
Succession
• Retreating
glaciers expose
new land surface
of till.
• Rate of retreat
ca. 65 km in 200
years
• Succession
follows broadly
predictable path
Nutrient
changes at
Glacier Bay
• The initial soil is nutrient
poor.
• Alder is an N-fixer, spruce
and hemlock are not.
• “forest floor” reflects N in
leaf and wood litter.
• Why is there a peak in
forest floor N at the
transition to sprucehemlock.
• Why does soil N decline
in the spruce-hemlock
zone?
AT least that has been the
accepted story..BUT!
• Fastie (1995, Ecology)
shows that alder may
actually slow the
succession through
competition.
• The succession to Sitka
Spruce was much faster in
the sites deglaciated in the
1780s-1840s than the later
sites.
• And hemlock has not
begun to grow at any site
that initiated after 1840.
Simulation showing nitrogen
inputs during 2ndry succession
Importance of alder (ALRU) as a nitrogen fixer
and Ceanothus (CEVE), early in succession.
Nitrogen sources
Rotmoos Glacier, Italian alps 1895
Rotmoos Glacier, Italian alps 1999
Rotmoos Glacier, Italian alps
• 1895 glacial tongue
evident in valley
• 1999 2km of retreat
evident
• Following is work
by Kauffman,
Ecology (2001)
First 50
yrs
• Sparse vegetation
means little local
productivity.
• Surprisingly, insects
are primarily predators
relying on
allochthonous (derived
from elsewhere)
sources of prey
5 yrs: Harvestmana glacial specialist
Predator.
10 yrs: 4 spp. Of
ground beetle,
occupying separate
niches.
20 yrs: assorted spiders
Abundant.
30 yr Centipedes, under
rocks
50 yrs: herbivorous
beetles as vegetation
density increases.
50-150 yrs
first
appearances
• As vegetation increases
in density a more normal
insect spectrum is
represented with food
chains supported by
herbivores. Detrital
cycle also evident.
System now
autochthonous
(productivity is local).
70 yr: millipedes
are important
decomposers.
100 yrs: 1 sp.
Of ant occupies
sunniest locations
140 yrs: Densest
Vegetation
supports
grasshoppers
Effect of succession on adjacent
waters
• Shading of margins
• As succession increases soil organic content will
also increase dissolved organic carbon (DOC),
e.g. humic acids and tannins.
• Leaching into waterways these chemicals color
the water and reduce transparency.
Dissolved Organic Carbon, e.g
humic acids influences aquatic
foodchains
DOC
Macro
Zooplankton
Spp.#
Low
2
Moderate
3-4
High
5
Mt St. Helens
example of
interference
• Lupins are N
fixers and were
colonists after the
Mt St. Helens
eruption.
• Lupin expansion
rapid at first but
slowed after a few
years.
Fagan & Bishop
(American Naturalist 2000)
Interactive
effects of
herbivory and
predation
• At expansion edge
Lupin expansion
limited by
herbivory.
• In center of lupin
range herbivory
limited by
predation.
This river is building a spit
• As the spit grows the
youngest vegetation
will be on the tip.
• Often a clear
succession, both in
terms of age and
species composition, is
evident.
Succession in streams &rivers
• Note the change in
size and species
composition beside the
channel
Can we make predictions about
the colonization process?
• Here is an island.
• Most plants are
going to arrive
either by wind,
sea, or bird/bat.
Krakatau case
study
• Krakatau is
an island
group in
Indonesia.
• A volcanic
eruption
sterilized
the islands
in 1883.
Krakatau: The best example of
primary succession
• Aug 27th 1883 Volcanic explosion
sterilized islands
• 2000 times the power of the bomb
dropped on Hiroshima.
• Generated tsunami that killed 36,000
people
• 100 m thickness of new ash coated the
islands…new land surface. A natural
laboratory.
• Colonization of plants and animals
documented since 1884.
Krakatau is west
of Java
Immediately after the eruption
• 1884: no plant life found, some blue-green algae growing
on ash.
• 1896: there were some coastal shrubs, scattered grasses
and shrubs in the interior.
• 1908: Interior a “parkland” of grasses and clumps of trees.
The succession
continues
• 1928-1932: forests close over the grassland.
• 1979-1992: forests changing in species composition.
Earliest trees now 60-80 years old.
Increase in species diversity
• Plant species
continue to
colonize the
islands.
• New plants
provide new
opportunities for
animals.
• Animals cannot
colonize until
foodplant is
present.
Pattern evident
in dispersal
mechanism of
arrivals
• At first wind and sea
dispersed species.
• Later trees dominated
by bird and bat
dispersed species.
• Wind dispersal still
brings orchids and
ferns.
Large pigeons and bats regularly
move between islands and
mainland
Development of structure
• As succession proceeds the physical structure of
the vegetation becomes more complex, offering
more niches. Trunk-cavities, vines, larger limbs.
• How would this affect recruitment?
Why structure matters
• The more layers in the canopy the higher the
animal diversity.Offer different feeding
opportunities.
• Trunk-cavities provide nest sites for birds and
insects.
• Vines provide food, cover and nest sites.
• Dead wood for decomposers
• Larger limbs better attachment sites for
epiphytes.
Biotic and abiotic influences
• Nitrogen likely to be
limiting nutrient early
in succession….why?
Source of recovery
will be different in
primary and
secondary succession
How habitat
quality
influences
succession
• Walker and Chapin’s
model considers the
importance of major
ecological factors in
succession in terms of
severe and favorable
landscapes.
Walker and
Chapin
Cont.
• Note how
differently
facilitation and
competition
influence
succession.